Flow sensors are utilized in a variety of fluid-sensing applications for detecting the movement of fluids, which may be in gaseous of liquid form. One type of flow measurement, for example, is based on thermal sensors, which can also be utilized to detect the property of a fluid. Thermal sensors may be implemented, for example, on silicon in microstructure form. For convenience sake, and without limitation, the term “flow sensor” can be utilized to refer to such thermal sensors. (See e.g. U.S. Pat. No. 6,322,247 FIGS. 10a-f, and U.S. Pat. No. 6,184,773, which are both incorporated herein by reference.). The reader will appreciate that such sensors may be also utilized to measure primary fluid properties such as temperature, thermal conductivity, specific heat (e.g. U.S. Pat. Nos. 5,237,523 and 5,311,447, which are both incorporated herein by reference.), and other properties; and that the flows may be generated through forced or natural convection.
A thermal-type flow sensor can be formed from a substrate that includes a heating element and one or more heat-receiving elements. If two such sensing elements are utilized, they can be positioned at the upstream and downstream sides of the heating element relative to the direction of the fluid (liquid or gas) flow to be measured. When fluid flows along the substrate, it can be heated by the heating element at the upstream side and the heat can then be transferred non-symmetrically to the heat-receiving elements on either side of the heating element. Since the level of non-symmetry depends on the rate of gas flow, and that non-symmetry can be sensed electronically, such a flow sensor can be used to determine the rate and the cumulative amount of the fluid flow.
Such flow sensors generally face potential degradation problems when exposed to harsh (e.g., contaminated, dirty, condensing, etc.) fluids, including gases or liquids that can “stress” the sensor via corrosion, radioactive or bacterial contamination, overheating, deposits or freeze-ups. The sensitive measurement of the flow, or pressure (differential or absolute) of “harsh” gases or liquids that can stress, corrode, freeze-up, or overheat the sensing elements is a challenge that is either unmet or met at great expense. Among the solutions proposed previously are passivation with the associated desensitization of the sensor, heaters to raise the temperature of gaseous fluids to be measured to avoid condensation or freeze-ups (or coolers to prevent overheating) at the expense of sensor signal degradation, cost increase and possible fluid degradation, or filters to remove objectionable particulate matter. Frequent cleaning or replacement and recalibration of the sensors are additional, but costly, solutions. Sensitive, membrane-based differential pressure sensors can be protected against contamination because no flow is involved, but they are less sensitive and more expensive than thermal microsensors, in addition to not being overpressure proof.
The measurement of liquid flow via thermal microsensors, especially of electrically conductive fluids, thus presents challenging problems in terms of electrical insulation, flow noise, chip corrosion, potential for leaks or structural integrity of the flow channel, and thermal measurement. The electrical contacts to the sensor chip generally should be insulated from each other so the resistance to electrical leakage is above approximately 20 MΩ to avoid interference with the sensing function. Some Si3N4 passivation films, for example, have pinholes; spin-on coatings of compounds that form glass or Teflon® films upon curing have not shown insulation beyond a few days of contact with salt water. (Note that Teflon® is a registered trademark of the E. I. Du Pont De Nemours & Company Corporation of 101 West 101 West 10th St., Wilmington, Del. 19898.) Even potting the wire-bonds in highly cross-linked epoxy led to either resistances dropping to, for example, 30MΩ and/or bond breakage if the epoxy became too brittle due to excessive cross-linking and/or thermal cycling. Additionally, the odd shape of the flow channel above the chip causes extra turbulence and corresponding signal noise. Another approach to providing electrical insulation for the electrical contacts and leadout wires is to move them out of the fluid-flow channel and contact area; however, such sidewise displacement adds real estate to the chip size and therefore to its cost.
Regarding structural integrity, a sensitive 1 μm-thick flow sensing membrane can easily break as a result of the stronger viscous and inertial forces that a liquid can exert on it. Such breakage has even been observed in cases of sharp gaseous pressure or flow pulses. Finally, with respect to thermal measurement issues, the heater temperature rise typically permissible in liquids (e.g., ≦20° C.) is much smaller than the one typically utilized in gases (e.g., 100-160° C.). The resulting, relatively small signal causes more significant increases in the effect of composition-, sensor- material- and temperature-dependent offsets, which can cause significant errors in the sensor flow readouts.
Based on the foregoing, the present inventors have concluded that the solution to the aforementioned problems lies appropriately in the “smart” application onto the sensing chip of a film that is strong enough to function as a protective barrier to the transfer of electrical charges and of molecular mass transfer but can be thin enough to enable transfer of heat to allow thermal measurements. The films may be fashioned of materials composed of inorganic compounds (even metals) or of hydrophobic or hydrophilic polymeric materials, as explained in further detail herein, which can result in operational flow sensors of high reliability, no electrical leakage, no fluid leakage by virtue of the non-intrusive character of the flow measurement, no corrosion, no fluid contamination, reduced flow noise and significantly reduced offset and drift problems.